Lens attachment design, wide angle and telephoto lenses.

Published on by Oleg Rybakovsky.

As discussed in previous posts, users of the Apple iPhone, Samsung Galaxy, HTC One, Nexus 5 and others smartphone can not interchange lenses to  alter focal length as is common with an SLR camera.

However, it is possible to obtain a variation in focal length with afocal attachments that fit in front of the native lens.  This is a bit like using a fixed lens camera to take photos through the eyepiece of telescope. Afocal attachments of the telescopic type are created by the separation of a pair of lens elements by a distance numerically equal to the sum of their focal lengths, giving the principal planes at infinity, i.e. the device has no focal length. Incident parallel light emerge parallel but the diameter of the beams is changed (see picture).

Tele_Wide Pic.png

The magnification in this arrangement is given by

m = f1 / f2 = feff / fcamera

when the camera is focused at infinity. For closer distances the actual magnification measured from the image size on the sensor depends on the focal length of the camera lens. The main problem of front lens attachments is that these lenses greatly increase the minimum focusing distance.

Using this method it is possible to create wide angle lens attachments for unique image capture. Typical requirements include:

·         Magnification - 0.6-0.7X (0.4-0.5X for super wide angle)

·         Resolution (center) - 250-300 lp/mm (close to resolution of native lens)

·         Resolution (corner) - 200-250 lp/mm

·         Distortion - < 1%

Below you can see how a lens with these parameters changes the captured image.

Wide-angle-iphone_1.jpg

It is also possible to use this design method to create a telephoto attachment. Typical requirements include:

·         Magnification - 2-2.5X

·         Resolution (center) - 250-300 lp/mm (close to resolution of native lens)

·         Resolution (corner) - 100-150 lp/mm

·         Distortion - < 1%

Design result:

As opposed to wide angle lens, the telephoto lens attachment decreases the field of view (FOV) of smartphone camera and presents a magnified image. This lens also differs from a macro lens attachment because of long focusing distance and use in a wide range of working distances.